Modern plastics are typically produced from petrochemical sources.
Plastics are generally made up of polymers, including long chains of repeating molecular units, or monomers. The vast majority of plastics are composed of polymers of carbon alone, or carbon in combination with oxygen, nitrogen, chlorine or sulphur in the backbone.
The properties of the polymer can be altered by introducing different functional groups into or onto the polymer backbone.
The history of plastic materials originated with the development of natural materials such as chewing gum and shellac. These materials however, require prohibitively expensive and intensive methods to isolate and manipulate the natural product.
Later developments included the use of chemically modified natural materials such as rubber and nitrocellulose, and later to the use of manmade molecules such as epoxy, polyvinylchloride and polyethylene.
The development of manmade plastic molecules has lead to a staggering worldwide increase in the use of plastics, for wide ranging purposes, including packaging, technology such as computers, cell phones and many household appliances. Plastic is cheap and easy to manufacture.
The main characteristic of polymers that allows it to be so widely-used is that some polymers can be thermoplastic (or plastic) and others can be thermosetting. Thermoplastic materials are deformable, they melt to a liquid when heated to a sufficient temperature and solidify into a solid state when cooled.
Most thermoplastics are high molecular weight polymers whose chains associate through weak Van Der Waals forces, for example in polyethylene; stronger dipole interactions and hydrogen bonding, for example in nylon; or stacking of aromatic rings, for example in polystyrene.
Thermoplastic polymers differ from thermosetting polymers. Whereas thermoplastic polymers can be repeatedly melted and cooled, thermosetting polymers, once formed and cured will not re-melt to allow re-moulding or re-use of the material.
Thermoplastic or thermosetting polymers can be formed into a desired shape by injection into moulds while in their liquid or fluid state and when cooled the shape of the mould is retained. In this way they can be easily be used to make a wide variety of complex shapes.
The manufacture of thermoplastics from petrochemical sources utilises the following general method:                1. drilling and transporting petroleum to a refinery,        2. refining crude oil and natural gas into ethane, propane and other petrochemical products,        3. cracking ethane and propane into ethylene and propylene using high temperature furnaces,        4. the addition of a catalyst to ethylene or propylene in a reactor, resulting in a powdered polymer,        5. combining the powdered polymer with additives (if required) in a continuous blender,        6. feeding the polymer into an extruder where it is melted,        7. cooling the melted plastic which is then feed into a pulveriser that cuts the cooled plastic into small pellets,        8. shipping the pellets to customers, and        9. customers manufacture plastic products from the pellets by various methods, including extrusion, injection moulding, blow moulding and rotational moulding.        
While the use of petrochemical sources to produce plastics is ongoing, it has a number of significant disadvantages, both to the environment and society. These include the following:
Firstly, plastics degrade very slowly. This leads to a high accumulation of unwanted and untreatable waste.
While methods are being trialled to increase the breakdown rate of plastics, such as the incorporation of biodegradable plastics or natural materials, such as starch is increasing, this in no way matches the worldwide consumption, and subsequent disposal of plastic items.
The high waste accumulation can also be off-set by recycling. However, recycling of plastics is not easy, and again includes a number of significant disadvantages. For example it is difficult to automate the sorting of plastic wastes, for example into plastic type or colour the use of manual sorting is very labour intensive. An additional complicating factor is that while many plastic containers are made from a single type and colour of plastic, which are relatively easy to sort, many other products such as cell phones, often include many small parts of different types and colours of plastics. In these situations the time and resources required to separate the plastics for recycling far exceed their recycling value.
A second significant disadvantage of standard plastics material is their effect on the environment. The long breakdown time means that if they get into the environment they can act to harm wildlife, for example the plastic rings which hold 6-packs of cans can easily get around the necks of, and strangle birds and other wildlife. The increase of plastic waste in oceans may also lead to the transport of small species from country to country, or continent to continent. This may lead to the introduction of invasive or unwanted pests into new areas.
Similarly, burning plastic material can in some cases release toxic fumes which can be harmful to those working or living in the area which again can be harmful and be difficult to get rid of.
Also, the manufacturing of plastics can often lead to large quantities of chemical pollutants.
A third significant disadvantage of petrochemical plastics is that petroleum resources are naturally limited. Therefore, in the future this is likely to lead to increased cost and decreased desirability of using these compounds on the current scale.
The problem with using petroleum based precursors in the manufacture of adhesives has been addressed by the development of a number of protein or soy protein based adhesives.
Proteins are natural biopolymers. The amino acids found in proteins offer many chemical interactions, due to the different functional side chains. Hydrogen bonds, ionic interactions, hydrophobic interactions and covalent disulfide bonds between these side chains give a protein its native structure. Proteins are versatile materials; the properties depend on the amino acid content and the modifications that are performed to improve specific properties. Reactive amino acids in proteins include the following: amide (15-40%), acidic (2-10%), neutral (6-10%), basic (13-20%), sulfur containing (0-3%) (De Graaf and Kolster, 1998).
In the materials industry these different side chains of proteins can be manipulated and used to add cross-linkers giving the material produced new mechanical properties. The processing of adhesives, films, coatings, or other protein based materials requires the breaking of intermolecular bonds (covalent and non-covalent), arranging the free protein chains into the desired shape, and then allowing the formation of new intermolecular bonds and interactions to stabilize the three dimensional structure. Cysteine, a sulfur containing amino acid, is found to be involved in non-disulfide irreversible covalent cross-linking (lysinoalanine and others) when proteins are placed under high temperature, which can become problematical in processing (Barone and Dangaran et al, 2006; Barone and Schmidt et al, 2006; De Graaf, 2000; Marion Pommet, 2003 and Singh, 1991).
Lysinoalanine is an un-natural covalent crosslink that occurs through the formation of dehydroalanine and reactive lysl residues occurs, in alkaline and heated systems. Cystine disulfide bonds form dehydro-residues in alkaline conditions, which are the reactive precursors for lysinoalanine. These non-disulfide covalent crosslinks once formed do not melt or exchange at high temperatures (Mohammed et al, 2000). There formation in a high protein system can prevent a flowable melt material forming.
The major disadvantage of using protein based sources in the manufacture of adhesives is that the lack adhesive strength and water resistance.
This issue has been addressed by using modified proteins such as soy, for example as described in WO 00/08110 which describes a method of using modified soy protein to provide a stronger, and more water resistant adhesive.
In the soy based adhesives described in WO 00/08110, the protein molecules are dispersed, and thus partially unfolded in dispersion. The unfolded molecules increase the contact area in adhesion of protein molecules onto other surfaces. The unfolded nature of the molecules also allows them to entangle each other during the curing process to provide additional bonding strength. Soy based adhesives overcome some of the problems associated with petroleum based products; they make use of soy proteins which are environmentally friendly and derive from soy beans which are more sustainable than petroleum resources.
The soy proteins in WO 00/08110 are modified with one or more modifiers, including, for example urea, guanidine hydrochloride, SDS (Sodium Dodecyl Sulphate), and SDBS (Sodium Dodecylbenzene Sulphonate) or a mixture of these.
The method disclosed involves mixing the modifiers, water and soy protein to form a slurry or dispersion. The modifiers act to unravel the proteins. After mixing the reacted dispersion can be immediately used as an adhesive, or can be freeze dried, milled into a powder and stored for later use after being reconstituted.
WO 00/08110 discloses reaction temperatures of between 10 to 80° C. under which the mixing is carried out, however, preferably the mixing process is undertaken at ambient temperature and pressure conditions.
Bovine blood has previously been used as an adhesive. The main use of this was in the manufacture of particle board (Francis, 2000)
One disadvantage of using protein polymers which decreases their usability, is that they lack the mechanical properties of petro-chemically derived polymers—this gives them unpredictable processing characteristics.
A further significant disadvantage of protein polymers is the price. Protein polymers are significantly more expensive than commodity petro-chemically derived polymers. This increased cost has in the past been sufficient to prohibit main-stream use of protein polymers in adhesives.
The use of soy protein for the manufacture of plastic materials also, given the high volume requirement for precursor material places a strain on the supply source. This may decrease the amount of soy for food based products.
Soy proteins also have the same disadvantages mentioned for protein polymers above, mainly the lack of mechanical properties and high price.
Extrusion work on proteins has previously been undertaken for zein and soy proteins. These were plasticised with oleic acid, glycerol or water. Extensive research has also been undertaken on corn gluten meal (mixture of various proteins found in corn). It was found that various additives were necessary to plasticise these proteins, and that the material had inferior strength compared to petrochemical equivalents.
It would therefore be desirable to provide a plastics material, and method of producing same from a high volume, low cost, sustainable and renewable protein source with sufficient mechanical properties.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.